A fundamental problem for behavioral neuroscience is to understand physiological control of individual variation in reproductive behavior and aggression. Progress in explaining sexually dimorphic behavior illustrates that this type of problem is most tractable in systems with discontinuous behavior variation. This application proposes to test the generality of the organization-activation model of hormonal control, which has been successfully applied to hormonal control of sexually dimorphic behavior, to situations of similarly discontinuous within-sex behavior variation. The most common example of the latter occurs in species with alternative male reproductive tactics. The hypothesis proposed predicts that the role that hormones play will depend on whether alternative male phenotypes are fixed or plastic. For fixed phenotypes, hormones should play a greater role during development (organization) than during adulthood (activation). The model system chosen, the tree lizard, has a fixed male behavioral polymorphism. Prior to sexual maturity, males permanently develop into one of two phenotypes. One phenotype is highly aggressive and territorial, whereas the other is less aggressive and appears to be nonterritorial and nomadic in nature. This proposal investigates the role of sex and adrenal steroids during early development in influencing the differentiation of these alternative male phenotypes. Specifically, correlational and manipulative endocrine experiments are combined to test the prediction that hormones act in a way analagous to organization by determining the type of hormone, the effective dose, the timing of hormonal treatment and the type of hormone metabolism necessary to effect phenotype differentiation. Together, the studies provide the first description of the role of hormones in within-sex behavioral differentiation, test the generality of the organizational-activational model and provide basic information useful in understanding behavioral variation, both continuous and discontinuous, in all species, including humans.